The following relates to the nuclear reactor arts, nuclear power generation arts, nuclear reactor safety arts, and related arts.
Electrical power grids comprise an interconnected network (i.e., “grid”) of power generation components, power transmission components, power conditioning components, and power consuming components (i.e., “loads”). Ordinarily, operation of the components is interdependent so that, for example, a power generation plant is designed to operate supplying power to a switchyard delivering power to a (cumulative) load that has known characteristics with statistically predicable narrow fluctuations.
These interconnections can be lost due to various failures. In particular, a “blackout” occurs when power supplied to the electrical power grid is abruptly interrupted. In such cases, it is known to operate a power generation plant in so-called “island mode” or “island mode of operation” where the plant is designed to accommodate the blackout condition.
In the case of a nuclear power plant, station blackout introduces radiological safety considerations. In some approaches, no island mode operation is attempted; rather, upon loss of switchyard power the reactor trips, control rods scram to shut down the nuclear chain reaction, and decay heat removal systems are brought online. Diesel generators and/or batteries are relied upon to supply power for the safety systems. This approach ensures safety, but subsequently requires a lengthy reactor restart process. Typically, power generation capacity is lost for days or longer. Moreover, the abrupt shutdown can stress the turbine and other components.
The Economic Simplified Boiling-Water Reactor (ESBWR) of GE-Hitachi (see http://www.nrc.gov/reactors/new-reactors/design-cert/esbwr/overview.html, last accessed Oct. 17, 2012) is designed to address station blackout by entering an island mode in which the switchyard breaker opens, a bypass valve dumps up to 110% of full steam load into the condenser, the BWR power output is reduced to about 40-60% over several minutes, and the (reduced) house electrical loads continue to be supplied by the turbine driven by the BWR. See “Advisory Committee on Reactor Safeguards ESBWR Design Certification Subcommittee”, Nuclear Regulatory Commission Official Transcript of Proceedings, Oct. 3, 2007 (Work Order No. NRC-1799). In other systems the bypass capacity is lower, e.g. 30% of full steam load. Id.
These approaches advantageously avoid reactor scram and subsequent reactor restart, but have certain other disadvantages. During the initial steam bypass into the condenser, the turbine loses steam and undergoes a transient, which can stress the turbine. The steam dump into the condenser also stresses the condenser. In the case of a BWR, there is substantial condenser capacity to accommodate the steam bypass, but a pressurized water reactor (PWR) typically has relatively less condenser capacity. It has been suggested that the ability in the case of a PWR to dump steam to atmosphere might be utilized (Id.), but venting to atmosphere raises other regulatory issues or overpressure alarms that would likely delay the operational restart of the PWR-based nuclear power plant with the power grid.
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